Aerostats, which are tethered lighter-than-air vehicles, are typically deployed for surveillance purposes. For example, it is known to provide aerostats with electrical monitoring equipment, such as radar, along geographical borders or areas of military engagement to monitor suspicious activities or opposing forces.
In operation, the aerostat carries electronic surveillance equipment or even temporary cellular communication systems. This equipment is in communication with ground stations so that appropriate personnel can make educated decisions about the area being monitored. The equipment or "payload" carried by the aerostat is typically quite heavy. Accordingly, the heavier the payload, the more helium is required to carry the payload. As will also be appreciated, the electrical equipment must be powered by an on-board generator or tethered power supply.
One known method for providing power to the payload aboard the aerostat is to carry an electrical generator on board. This provides all the necessary power needs of the aerostat in a somewhat efficient manner. Unfortunately, electrical generation equipment is quite heavy and decreases the amount of surveillance equipment that may be carried by the aerostat. Another drawback of employing an on-board power generator is the reduced "availability" of the aerostat. In other words, the generator typically only has enough fuel to power the surveillance equipment for about 5 to 10 days. At the end of this time, the aerostat must be retrieved, serviced, and then re-deployed. This allows the suspicious activity or enemy forces to be unobserved for a significant period of time.
In order to increase the availability of aerostats, ground-based power systems have been developed wherein the power is delivered from a power station via the tether to the aerostat. Another benefit of such a system is that additional surveillance equipment may be installed in place of the now ground-based generator. Moreover, any problems with the generator can be dealt with on the ground instead of having to retrieve the aerostat anytime the generator has a malfunction.
Known tether constructions for providing power to deployed aerostats consist of three electrical conductors centered within a protective sheath. Strength members, such as Kevlar.RTM., are provided in the interstices of the three conductors and about the periphery thereof. Surrounding this core of electrical conductors is a dielectric material which is then covered by a metallic braid to reduce or eliminate any electromagnetic interference that would otherwise be easily detected by opposing forces. The metallic braid is also employed to conduct induced lightning current to ground. Detection of the aerostat would likely result in movement of the suspicious activity or attacks on the aerostat or its ground station.
Although the tether-based power delivery system is more effective than the on-board power delivery system, it will be appreciated that there are significant drawbacks to such a system. In particular, the three-conductor system, which only provides three-phase power, has limited power availability and provides only a transmission frequency of 400 hertz. It is estimated that there is a 50% power loss in transmitting the electrical energy from the ground-based station to the equipment aboard the aerostat. Yet another drawback of the tether-based power delivery systems is that the conductors are centered within the tether and may function as the load members. This subjects the conductors to significant tensile and shearing stresses from the wind and the winches that deploy and retrieve the aerostat. Still yet another drawback of the currently known tether system is that the metallic shielding braid significantly increases the weight of the tether and therefore, reduces the payload that may be carried by the aerostat. By employing low frequency power, heavy magnetic components aboard the aerostat must be used. Accordingly, the payload of the aerostat is further reduced. Yet another drawback of the known tether construction is that if lightning strikes the strength members, they can be severed and the aerostat may be released from the tether and lost. And in a worst case scenario, the aerostat and its payload may fall into enemy hands. Still yet another drawback of the present construction is that if one of the three phases, that is, one of the conductors, is removed from operation, unbalanced power is delivered to the equipment, possibly causing shut down or at least radiated EMI. As such, the aerostat must be retrieved and the tether replaced.
Based upon the foregoing, it is evident that there is a need for a lighter tether that allows for an increase in the payload carried by the aerostat. Moreover, there is need for a tether which provides more power to the payload, provides redundancy and improved power delivery and is configured so as to virtually eliminate electromagnetic interference emanating therefrom.